Fluidized bed incinerators have heretofore been used for incinerating municipal refuse. Where municipal refuse is burnt in a fluidized bed incinerator, refuse is consecutively charged into it. In the great majority of cases, a tremendous amount of trash is charged in one mass with different articles entangled with each other and forced into an agglomerated mass. Fluidized bed incinerators have a rather higher rate of combustion than other types of incinerators, and also exhibit the advantage of providing in some cases a condition in which matter is well burnt. Paradoxically, this causes the drawback that, once the matter to be incinerated has been charged into the fluidized bed, it may be burnt within a few seconds because of the high combustion performance. For this reason, if the feeder used to feed the matter to be incinerated into the furnace is inferior in terms of maintaining a constant feed rate, there will be a problem in that any variation in the amount of matter to be incinerated which is charged into the furnace will directly lead to fluctuations in the concentration of oxygen contained in the combustion gas.
If the concentration of oxygen contained in the discharged combustion gas is approximately 5% or less, the critical amount depending on the type of fluidized bed incinerator, carbon monoxide and carbon hydrides such as methane, ethylene, propylene, acetylene and benzene will be discharged without being completely burnt. Thus, materials such as ammonium chloride and ammonium hydroxide will be generated, which lead to the emission of white smoke from the stack. Because fluidized bed incinerators exhibit high combustion performance, combustion can be effected so long as the superficial velocity of the fluidizing air is adequate for fluidization even if the theoretical air ratio of the fluidizing air blown into the fluidizing medium is smaller than 1. In order to inhibit the generation of unburnt gases such as carbon monoxide, however, the air ratio is increased. In some cases, extra air is fed beforehand so as not to reduce the concentration of oxygen even if the supply of the matter to be incinerated is increased to cope with the risk that the ability of the feeder to provide a constant feed rate will deteriorate.
The amount of air blown into the furnace is, at the maximum, twice as much as the theoretical quantity of air, depending on the ability of the feeder to ensure a constant feed rate. Even in this case, however, the various items of refuse are entangled with each other to form large agglomerated lumps, particularly when dealing with the municipal trash. Finally, a so-called massive drop takes place, leading momentarily to a lack of oxygen, and thus unburnt gas (not yet burnt) like carbon monoxide is sometimes discharged from the stack.
In prior art methods of inhibiting the discharge of unburnt gas, it has been necessary to improve the capability of the feeder to provide a constant feed rate. In addition, as disclosed in, e.g., Japanese patent application No. 223198/1984 (Japanese Patent Laid-Open No. 100612/1986), a measuring means may be provided for the purpose of measuring the amount of matter for incineration actually charged, allowing that amount to be reduced by lowering the rotational speed of the feeder when it is sensed that the amount of matter for incineration charged was increased.
Another method has been adopted whereby secondary fresh air is blown, into the incenerator when it is sensed that there has been an increase in the amount of matter charged or a shortage of oxygen has occurred.
Where a feeder is utilized in the conventional mode of inhibiting the discharge of unburnt gas, the potential for improvements in its ability to provide a constant feed rate is limited, with the result that expensive feeders have to be used.
The method disclosed in Japanese patent application No. 223198/1984 involves the use of a device for measuring the amount of matter charged. Use of this device, however, results in a shortage of oxygen, because the matter for incineration dropped into the furnace is immediately burnt. Secondary fresh air is blown into the furnace to compensate for this shortage, at which time the volume of exhaust gas is increased because of the introduction of the secondary air as well as the increase in exhaust gas resulting from the intensive combustion. Thus the pressure within the furnace becomes positive. When this positive pressure is sensed, an inlet damper of an induction fan is opened to normalize the furnace pressure. Therefore, if a good deal of matter for incineration is charged, the furnace pressure fluctuates, gas is injected through a exhaust gas duct flange and an ash-discharging rotary valve because of the positive pressure within the furnace, and this results in powdery dust contained in the exhaust gas being scattered which leads to a dusty environment in the plant.
Methods of controlling secondary fresh air to maintain the concentration of oxygen contained in exhaust gas at a certain level also involve the following inherent problems. Since the combustion rate of a fluidized bed incinerator is quite high, any fluctuation in the rate at which matter for incineration is fed into the furnace is directly reflected as unevenness in the rate at which gas is discharged, and hence the drawback mentioned above will also be encountered. A further problem is that the presence of a large amount of combustion air involves the provision of a large combustion fan and a large gas discharge inducing fan, which in turn requires that much power is consumed in driving these fans. Moreover, as the volume of gas discharged fluctuates the processing equipment installed for handling this gas which includes an exhaust duct, a gas cooler and an electric dust collector needs to have a large capacity to deal with the maximum possible flow of gas. This means that both the size of the incineration equipment and the total cost of construction are excessive.
In a conventional fluidized bed boiler, particularly in a fluidized bed boiler used for power generation, the quantity of coal fed into the boiler is varied to accord with any fluctuation in load, as is disclosed in Japanese Patent Laid-Open Publication No. 1912/1984. Whenever the quantity of fuel being supplied is increased the rate of combustion is controlled by a method of regulating the feed rate of fluidizing air fed from the lower portion of the fluidized bed so that the temperature of the fluidizing medium in the fluidized bed is not in excess of a predetermined value. Even with use of this combustion control method, it has been impossible to inhibit the discharge of unburnt gas without causing fluctuations in the respective amounts of combustion air and exhaust gas while at the same time restraining sudden fluctuations in combustion rate, especially when the amount of matter to be incinerated charged into the furnace varies in a fluidized bed incinerator for incinerating such matter as municipal refuse, since such refuse comprises a mixture of various constituents differing from each other in bulk, configuration, combustibility and calorific value.
It is to be noted that the combustion rate is herein given by: calorific value (kcal/kg) x volume of material for incineration (amount of matter for incineration) (kg/time).
The present invention has been conceived in the light of these circumstances and it is a primary object of the present invention to obviate the above-mentioned problems incidental to the prior art by providing a combustion control method for application to a fluidized bed incinerator which is capable of inhibiting the discharge of unburnt gas without increasing the respective amounts of combustion air and exhaust gas and without any need for an expensive feeder having a high capability to ensure a constant feed rate even if matter to be incinerated such as coal, municipal refuse, industrial scraps or mixtures thereof with differing calorific values, rates of combustibility, configurations and bulk volumes is charged into the incinerator and the amount of matter so charged fluctuates.